<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  手机号码：<input type="number">
  <button>发送</button>
  <script>
   var btn = document.querySelector("button")
   btn.onclick = function()
   {
     btn.disabled = true
     var t = 5
    var id= setInterval(function(){
       t--;
       btn.innerHTML="还剩" + t + "秒"
       if(t==0)
       {
         clearInterval(id)
         btn.disabled = false
         btn.innerHTML ="按钮"
       }
     },1000)
   }
  </script>
</body>
</html>